Combining isotope ratios for provenancing Viking Age iron artefacts in the British Isles: a pilot study

Stable and radiogenic isotope analysis – particularly using lead isotope analysis (LIA) – has previously been shown to be a useful tool for the provenancing of ancient metal artefacts of silver and copper and its alloys, but less progress has been made in the provenancing of iron artefacts, despite their importance and frequency in the archaeological record. In this pilot study we investigate for the first time the possibilities of iron isotope analysis in combination with trace strontium isotope analysis and LIA for the provenancing of iron objects believed to be from the Viking Age in the British Isles. Previous studies have shown that analysis of each of these isotopes can contribute to provenancing iron artefacts, but they are not individually resolutory. In this proof-of-concept study, we examine the Fe, Sr and Pb isotopes of 7 artefacts believed to derive from the Viking Age: 3 from Meols – a former Viking seaport on Wirral and 4 samples from the probable location of the AD 1066 Battle of Fulford in North Yorkshire. We also examine an additional artefact of unknown antiquity from Bebington Heath – a possible location of the AD 937 Battle of Brunanburh. Although the pilot data set is too small to make definitive conclusions, it has paved the way for a fuller study involving 100 samples (including 30 from the former Viking camp of Torksey, Lincolnshire) funded by the NEIF fund of the UK National Environmental Research Council. The high range of 87Sr/86Sr values in the present data set of 8 is beyond what would be expected for bog iron (with a cut-off ∼ 0.709) and suggests that mined ore was being used, a preliminary conclusion supported by the narrow range of Fe isotope data.


Introduction
This pilot study explores the potential of a combination of stable and "radiogenic" isotopesthe stable end products of radioactive decay seriesof iron (Fe), lead (Pb) and strontium (Sr), present in iron artefacts, to provenance iron objects from the Viking Age.
Isotope analysis for provenancing is based on the principle of the measurable variation of isotope ratios of elements which are transmitted from the source to the metal.The four stable isotopes of iron are 54 Fe, 56 Fe, 57 Fe and 58 Fe with respective natural abundances of 5.8%, 91.7%, 2.2% and 0.3%.The variation of 56 Fe/ 54 Fe and 57 Fe/ 54 Fe ratios relative to an international standard may be used to ngerprint different iron sources for comparison with ancient objects.The four naturally occurring isotopes of strontium are non-radiogenic 84 Sr, 86 Sr and 88 Sr, and radiogenic 87 Sr derived from the radioactive decay of 87 Rb.The ratio 87 Sr/ 86 Sr can be used to determine the source of various archaeological artefacts. 1Lead has four isotopes: stable 204 Pb and radiogenic 206 Pb, 207 Pb and 208 Pb produced by the radioactive decay of 238 U, 235 U and 232 Th, respectively, with different rates of radioactive ingrowth.These characteristics make Pb a powerful tracer for archaeological artefacts. 2he provenancing of archaeological iron artefacts is not only useful for understanding the source of the raw materials, but also, by extension, for understanding trade routes or the migration of the people who carried them.Until recently, however, the focus has tended to have been on copper, copper alloy and silver artefacts 2 despite the large numbers of iron artefacts found on many sites of archaeological importance.This is because lead isotope analysis (LIA) has shown itself to be a very effective tool for the provenancing of ancient metal artefacts of copper and its alloys, silver and lead 2 but its usefulness for the provenancing of iron artefacts has not been fully established. 3,4ecent research has proposed the use of LIA together with trace element patterns of slag inclusions, 3 LIA in combination with strontium (Sr) isotopes, 4 or an alternative combination of osmium (Os) and Sr isotopes. 5Moreover, it has recently been proposed that Fe isotopesbased on the natural variability of iron orespreviously considered for geological or biomedical applications, [6][7][8][9] may be useful for provenancing iron artefacts, [10][11][12] particularly when used in conjunction with trace element analysis. 12The methodology has been successfully applied to elucidating the possible origins of 12 Roman iron barsdating from between 100 BC to AD 100discovered in a shipwreck off the coast near Les Saintes-Maries-de-la-Mer (Bouches-du-Rhône), in south-eastern France. 11,12he main limitation for all isotopic and elemental tracers is the potential overlaps of composition between distinct sources.Combining several tracers whose variations are not correlated, as we do here, can provide complementary information (geological origin, nature of the source), and is therefore the most promising approach.One of the great advantages of isotopic analysis when dealing with archaeological materials is that it causes very little damage to ancient objects, compared to the conventional approach of trace element analysis of slag inclusions.
The approach taken by previous iron-provenancing studies has been to concentrate on analysing material from a single archaeological site and the ores which are thought likely to be the source of the iron used to make them [3][4][5] rather than comparing artefacts from a selection of archaeological sites.Conversely, our pilot study takes the approach of examining material from three different archaeological sites.Iron ores are extremely widespread throughout the British Isles 13 and consequently in many cases it is difficult to identify the likely sources of the iron ore used at specic archaeological sites, especially in later periods when iron circulated more widely.
In the British Isles, aer the collapse of Roman Britain in the 4 th century AD, it is thought that iron was initially obtained from bog ironwhich consists primarily of iron oxyhydroxides, commonly goethite (FeO(OH))but that it was increasingly replaced by mined iron ore in the later part of the Anglo-Saxon period and into the Viking Age. 14 Bog iron is a form of impure iron deposit that precipitates in bogs or swamps as a result of the chemical or biochemical oxidation of iron carried in solution.
A further driver of this pilot study has been the desire to answer important questions concerning the origins of iron artefacts found in close proximity to each other at the former Viking Age seaport of Meols on Wirral, north-west England and the precise location of two Viking Age battles (Fulford AD 1066 and Brunanburh AD 937).

Meols, Wirral
In AD 902, Irish Chronicles known as the three fragments describe a settlement of "mass migration proportions" in Wirral, a small peninsula between Wales and Liverpool in north-west England, of Norse Vikings expelled from their former base of Dublin in Ireland. 15,16This was a peaceful settlement and was the result of an agreement between the Norse leader Ingimund, and Aethelaed, Queen of the Mercian English.The peninsula is full of Norse names such as Thingwall (Old Norse 5ing völlr -"Assembly Field") in the centre and its seaport at Meols (Old Norse melr -"sandbank").[17][18][19] 1.2.Brunanburh, AD 937 The location of Brunanburh has been debated for centuries: 20 it was the site of a key battle between a combined army of the Anglo-Saxon kingdoms of Wessex and Mercia, and a northern alliance of Scots and new wave of Norse Vikings coming from Ireland, with later reports of Icelandic Vikings ghting on the side of the Anglo-Saxon forces.It was a battle for the domination of Britain, to decide whether Britain was to be united under a single powerunder the Saxon leader Aethelstan (nephew of Aethelaed)or would remain as discrete entities (a question which remains with us today).
1][22] As a consequence, many scholars believe that the battle of Brunanburh was fought on the Wirral peninsula: a site for the battle (Bebington Heath) and Dingesmere (the River Dee coastline around Meols) has also been suggested.Increasing numbers of Viking Age artefacts are being found at Bebington and if isotope analysis can conrm that a signicant number originated from Scotland, this would identify Brunanburh with respect to the other conicts between Vikings and Anglo-Saxons that took place in the area.

Fulford, AD 1066
Fulford (North Yorkshire) was the location of a battle in AD 1066 between Norse invaders and Anglo-Saxons, immediately before the better known battle of Stamford Bridge.The archaeological material consists of iron objects found by excavation of a number of short-lived iron-recycling sites that were abandoned by the Norse victors at Fulford when they were defeated at Stamford Bridge ve days later. 23,24The Fulford evidence suggests that serviceable weapons were gathered and removed while damaged metal artefacts were processed, leading to the hypothesis that the nds from the site attest to post-battle metal recycling.However, the interpretive value of scattered battle-eld nds is currently limited by the available science.If it was possible to provenance or simply prole nds this might provide a link to the combatant's source of weaponry, and possibly associate the found battle fragments to help conrm battle descriptions.

The pilot project
The considerable advances in instrumentation for analysis of isotope ratios, together with the development and population of databases, 25 fuelled by a recent joint meeting of the Royal Society of Chemistry with the Society of Antiquaries, 25 now makes resolution of these and other related issues a realistic possibility.For this pilot study we examined seven artefacts believed to be from the Viking Age (Fig. 1, Table 1): three from Meols on Wirral, courtesy of the Grosvenor Museum, Chester, and four from Fulford, near York, courtesy of the Fulford Battleeld Society.As a working hypothesis, it is necessary to assume that weapons/objects were made from 'rst-generation' iron, forged from a single furnace-bloom.Conversely, if metal from different places was melted and mixed together, then provenancing becomes more difficult as the isotope signatures of the artefacts will not correspond to those of the individual ore sources.We also examined an additional artefact of unknown antiquity from Bebington Heath (also Wirral)a possible location of the AD 937 Battle of Brunanburhcourtesy of Wirral Archaeology CIC (Community Interest Company).

Materials
The three objects from Meols, Wirral (Fig. 1) were found in close proximity to each other at low tide in sand.The four from Fulford were found by excavation.The additional wrought iron object of unknown antiquity found at Bebington Heath, also on Wirral, was also found by metal detectorists.It was originally thought to have been a pommel from a sword but subsequent X- ray analysis 26 has shown this not to be the case.We refer to it simply as a Bebington Heath "artefact".

Drilling and dissolution
A small area of each of the artefacts was abraded clear of weathering/corrosion using a diamond burr and then each sample was drilled, in order to obtain a sample of clean iron (30-50 mg for each object).To minimize the risk of damage to the objects, samples were held against a resilient pad and a handheld drill was used.Each extracted sample was split in half; one aliquoted for Fe isotope analysis at the GET laboratory (Toulouse, France) and the remainder for Sr and Pb isotope analysis at the British Geological Survey.
Aliquots dedicated to Fe isotopic analyses were weighed in clean Teon beakers and digested using a mixture of bi-distilled 6 M HCl and 15 M HNO 3 , together with Merck supra-pure HF acid on a hot plate at 120 °C.Samples were then taken to dryness and re-digested in distilled 6 M HCl at 120 °C until no solid particles remained in the solution.Once totally dissolved, the Fe content of the samples was puried in a single step chromatography on an anion exchange Biorad© AG1-X4 resin in HCl medium. 27he samples for Sr and Pb analysis were transferred to a clean laboratory (class 100, laminar ow).30-100 mg of 84 Sr tracer solution was added depending on sample size, dissolved in Teon distilled 8 M HNO 3 .Aer evaporation to dryness, the samples were converted to bromide form by addition of 0.5 M Ultrapur© HBr.Lead was collected using Eichrom© AG1X8 anion resin.The residue from this separation was evaporated to dryness and converted to chloride form by addition of Teon© distilled 6 M HCl.The strontium Aer purication, the Fe isotopic composition of the samples was determined by high resolution multicollector inductively coupled plasma mass spectrometry following the procedure described by Poitrasson 28 and Milot et al. 10 The Faraday cup conguration is given in Table 2(a).In addition to 54 Fe, 56 Fe and 57 Fe, the isotopes 53 Cr, 60 Ni and 61 Ni were measured to correct the Cr isobaric interference on mass 54, and for mass bias correction with Ni isotopes.Each sample was bracketed by analysing IRMM-14 reference material in the analytical sequence.The mass bias was corrected by combining standard-sample bracketing and by a daily regression method using the Ni added in every sample and standard solutions.In addition, we measured the composition of an in-house haematite standard (ETH haematite from Milhas, Pyrenees Mountains, France) every six samples for quality control.The Fe isotopes composition of the samples and standard are expressed in delta notation, 10 in per-mil (&), relative to IRMM-14 (for example for 57 Fe/ 54 Fe ratio: d 57 Fe = {( 57 Fe/ 54 Fe) Sample / ( 57 Fe/ 54 Fe) IRMM-14 −1} × 1000).Each sample was analysed at least three times (n = 3 for ME04, WA10, FUL01, FUL09, FUL19, and n = 6 for ME02, ME03 and FUL14) and the analytical uncertainties in Table 3 are reported as 2SE (standard error).
Because of the mass dependent fractionation of iron isotopes in nature (d 57 Fe = d 56 Fe × 1.5), the use of d 57 Fe or d 56 Fe makes no difference for discussing the results.However, we preferentially report d 57 Fe in the discussion part of this paper as it yields greater variation than d 56 Fe because of the 3 atomic mass units difference between 57 Fe and 54 Fe.The 21 measurements of the ETH haematite displayed d 57 Fe of (0.761 ± 0.082)& and d 56 Fe of (0.517 ± 0.060)& (2SD).This is consistent with previous measurement of the same ETH standard reported by Sossi et al. 29 who found d 57 Fe = (0.753 ± 0.094)& and d 56 Fe = (0.514 ± 0.049)& and Ratié et al. 30 who found d 57 Fe = (0.762 ± 0.083)&.This indicates the good quality of our results.
2.3.2.Sr isotopes.Strontium was loaded onto a single Re lament following the method of Birck 31 and both the isotope composition and strontium concentrations were determined by Thermal Ionisation Mass Spectroscopy (TIMS) using a Thermo Triton (Thermo Scientic, Bremen, Germany) multi-collector mass spectrometer, with Faraday cup conguration given in Table 2(b).The international standard for 87 Sr/ 86 Sr, NBS987, gave a value of 0.710 259 ± 0.000020 (2SD, n = 8) during the analysis of these samples and the data are normalised to the accepted value of 0.710 250.The international rock standard (Columbia River Basalt BCR-2) gives the following reproducibility through sample dissolution, column separation and mass spectrometry analysis: 87 Sr/ 86 Sr = 0.705 016 ± 0.000026 (2SD, n = 26) during the analysis of the samples in this study.This compares very favourably with the accepted value of 0.705 013 ± 0.000010 (2SD, n = 13)see ref. 32.
2.3.3.Pb isotopes.Pb isotope analysis of the samples was conducted using a Thermo Scientic (Bremen, Germany) Neptune Plus MC-ICP-MS (multi-collector inductively coupled plasma mass spectrometer).This mass spectrometer is tted with the Jet interface, in which enhanced sensitivity is achieved a SE: standard error; SD: standard deviation.T is the 'Model Age' parameter (in millions of years). 35hrough the use of a large volume interface pump (Pfeiffer On-Tool Booster 150) in combination with the Jet sampler and X skimmer cones.Prior to analysis, each sample was appropriately diluted (using Teon distilled 2% HNO 3 ) and spiked with a solution of thallium (Tl), which is added (in a ratio of ∼1_Tl : 10_Pb: this provides an intensity that can be measured accurately while minimising the use of highly toxic thallium) to allow for the correction of instrument induced mass bias.Samples were then introduced into the instrument via an ESI 50 ml min −1 PFA micro-concentric nebuliser attached to a desolvating unit, (Cetac Aridus II).All Pb isotopes of interest were simultaneously measured using the Faraday (see, e.g.Evans et al. 33 ) cup conguration detailed in Table 2(c).
The acquisition consisted of 50 ratios, collected at 8.4 second integrations, following a 60 second de-focused baseline measurement made at the beginning of each analytical session.
The precision and accuracy of the method was assessed through repeat analysis of NBS 981 Pb reference solution, (also spiked with Tl).Data are corrected (normalised) relative to the known values for this reference, taken from Thirlwall: As with strontium isotope analysis, the international rock standard (Columbia River Basalt BCR-2) gives the following reproducibility through sample dissolution, column separation and mass spectrometer analysis for lead isotopes: 206 36 This latter method provides a more accessible method of providing reference elds that relate to major tectonic related mineralization events. 41

Fe isotope analysis
Values for d 56 Fe and d 57 Fe are shown in Table 3, and Fig. 2. Signicant variation of Fe isotopic composition appears among the 10 iron objects analysed.The samples from Meols have a very narrow composition range comprised between −0.139 and −0.076& for d 57 Fe.Those from Fulford display a broader range of d 57 Fe values, from −0.269 to 0.125&.The Fe isotopic composition of the artefacts from Meols, which is relatively homogeneous, might be taken to indicate a single ore source, but the Sr and Pb isotope data for these objects do not show close grouping: this shows that isotope ratio data from more than one element is necessary before making robust conclusions on a common provenance of a group of objects.In contrast, the objects from Fulford display signicantly distinct Fe isotopic compositions, which likely indicates that they were made of iron from a variety of ore sources.Although important overlaps occur between the sites, a striking point is the narrow total range of isotopic variability of these 8 objects (0.421& for d 57 Fe).This may indicate hydrothermal-derived ore sources for these objects, instead of sedimentary iron ores which would display more fractionated compositions. 12In particular, these results do not argue for a source from bog iron ores from eastern England, since the high variability previously measured in such ores (about 4& for d 57 Fe measured in bog iron from Germany by Rose et al. 37 ) would likely to have been reected in these objects.Unfortunately, that is as much as we can say at the present time as there is a sparsity of comparative data for the British Isles and Scandinavia, although commercial analysis facilities are now available 38 .

Sr isotope analysis
Fig. 3 gives the distribution of the minor strontium isotope ratio 87 Sr/ 86 Sr as a function of strontium concentration for the eight   iron objects.The Sr concentrations are generally low, between 0.5 and 5 ppm (mg g −1 ) similar to the range (>15 ppm) noted for iron artefacts from south-east Turkey. 39The Sr isotope compositions are variable and range between 0.7091 and 0.7153.There is no correlation between the Sr composition and the sites at which they were found.
There is little published data on the Sr isotope composition of iron in Northern Europe with which to compare the data from the weaponry.However, four samples have been analysed from iron production sites along the River Foulness, near Holme-on-Spalding-Moor, East Riding of Yorkshire (Table 4).All the Sr concentrations are below 0.7097.These bog iron and slag sample values are consistent with an ore formed in coastal wetland or rain supplied bogs which are dominated by marine/rainwater values close to 0.7092. 40he results from the artefacts display a far wider range of Sr values than can be accounted for from coastal wetlands and rain dominated bogs (Fig. 3b).A number of possible reasons for the data range can be posited that will be the focus of future studies beyond this pilot, namely (1) their ore comes from a geologically deposited source related to mineralization (2) that there are bogs with water sources that are not predominantly rainwater, but possible aquifer on rocks, with more radiogenic source, or (3) the process of making the weaponry involved the addition of a component with radiogenic Sr values.

Pb isotope analysis
A better picture comes from trace Pb isotope analysis of the iron objectsthis is primarily because, in contrast with the Fe and Sr isotope data, the databanks of Pb isotopes for the British Isles are much better populated, based on rock, ores and faunal sources. 32,33Fig. 4 gives bivariant diagrams of 207 Pb/ 204 Pb versus 206 Pb/ 204 Pb minor isotope ratios for our 8 objects.
In a recent study, Evans and colleagues 33 explored the distribution of Pb isotopes throughout the British Isles, taking advantage of the fact that unlike for strontium isotopes whose distribution is affected by underlying rocks, for lead there is a tectonic boundary between the Solway Firth and Berwick on Tweedthe Iapetus Suturewith clear isotope signatures appearing to the north and south of the suture which closely maps the current boundary between Scotland and England.Fig. 5a shows the contoured map of 206 Pb/ 204 Pbwith distinct demarcationsand Fig. 6a shows the distribution of the related 238 U/ 204 Pb (m values), which shows even greater resolving potential.Scottish Pb mineralization generally has a signicantly different "older" isotope signature 39 separated by the socalled Iapetus Suture. 41e can use this variation in 206 Pb/ 204 Pb or the derived parameter m in one of two ways.Firstly, in terms of a bivariant plot of 208 Pb/ 204 Pb (y-axis) versus 206 Pb/ 204 Pb (Fig. 5b); or equivalently following Albarède et al. 36 and Evans et al. 33 plotting m (y-axis) versus the 'Model Age T' parameter (Fig. 6b).

Concluding remarks
The limitations of trying to make conclusions about the provenancing of 8 objects (from 3 sites in the British Isles) based on current databases are all too clear.Our pilot study shows patterns in the data, but the number of samples was much too low to understand the signicance of this nding, and particularly with the iron and strontium isotope data we await the development of databases against which to compare our own samples.The high range of 87 Sr/ 86 Sr values, beyond what would be expected for bog iron (with a cut-off around 0.709), suggests that mined ore was being used, a preliminary conclusion supported by the Fe isotope data, and that the Sr in some of the samples is likely to come from sources other than bog iron.And this study cannot exclude the possibility that some of the iron objects could be derived from direct mineralization deposits rather than secondary bog precipitation: that question will not be addressed until we have more direct measurements of Sr in bog-iron samples.
The more extensive work on lead isotopes across the British Isles in particular, show clear differences in published data for Scotland due to the Iapetus Suture and the known uranium depletion of the old Laurentian basement which underlies much of Scotland. 33This is particularly important in trying to address the question of whether any iron objects found at Bebington on Wirral are unequivocally associated with the lost Battle of Brunanburh.The current "test" object -WA10 of unknown antiquity clearly is not, and may not be Viking Age at all.It also needs to be established what the controls from Ireland and Scandinavia are like, to enhance the diagnostic plots such as Fig. 5b and 6b, which are likely to be further rened as the databanks grow.
Nonetheless this pilot study has paved the way for a more extensive study where we continue to analyse a much larger number of objects (90) from Fulford, Bebington Heath and the former Viking army encampments of Torksey (Lincolnshire), and Aldwarke (South Yorkshire) 42,43 together with 10 samples of bog iron and slag from the Foulness Valley (East Riding of Yorkshire).The more samples of bog iron from this latter important and well-documented 44 sourceknown to have been exploited since the later rst millennium BCwill be included in the extended study in order to exemplify the isotope signal of bog iron.We may then be able to start to obtain some general ideas as to the provenance of the artefacts.The work on iron, strontium and lead isotopes will also be reinforced by trace osmium isotope analysis 45 and investigating the relative concentrations of a range of trace elements, including: phosphorus (P), manganese (Mn), barium (Ba), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn) and arsenic (As), as well as of course strontium (Sr) which has been included as part of the present pilot study.Iron, strontium, lead and their isotopes thus far seem to tantalisingly offer the best hope for separating iron provenances, but more objects need to be found and the databases need to be extended. 46Indeed, this work is being undertaken in parallel with growth of the appropriate databases, recently stimulated by a joint meeting of the Royal Society of Chemistry and the Society of Antiquaries (UK). 25

Fig. 1
Fig.1Iron objects analysed: Viking Age objects from Meols on Wirral, courtesy of the Grosvenor Museum, Chester (an axe-head ME02, a bent spearhead ME03 and a tool or spearhead ME04); Viking Age objects from Fulford, York, courtesy of the Fulford Battlefield Society (a repaired sword Full 01, a tanged arrowhead Full09, an axe-head Full14 and a planishing anvil Full19); artefact from Bebington on Wirral, courtesy of Wirral Archaeology CIC (WA10).

Fig. 3
Fig. 3 Strontium isotope analysis (a) Sr concentration plotted against 87 Sr/ 86 Sr composition for the artefacts, and (b) same plot but with the object types.The uncertainties in concentration are ±1.7%2SD and the uncertainties in the isotope ratio are ±0.0000202SD based on the reproducibility of NBS 987 standard.This is < the size of the symbols used.

Fig. 5
Fig. 5 (a) Distribution of 206 Pb/ 204 Pb within Britain based on data from mineral sources.Adapted from Evans et al. (2022) 33 and reference cited therein.(b) Plot of 208 Pb/ 204 Pb vs. 206 Pb vs. 204 Pb with the contour zones from (a).

Fig. 6
Fig. 6 (a) Distribution of 238 U/ 204 Pb (m) within Britain based on data from mineral sources.Adapted from Evans et al. 33 and references cited therein (b) plot of m = 238 U/ 204 Pb versus the Model Age parameter T (in millions of years).

Table 1
Sample sites for the objects

Table 2
Cr and 202 Hg were measured for the correction of the isobaric interference of 54 Cr on54Fe, and 204 Hg on 204 Pb, respectively.

Table 4
Strontium isotope results for bog iron/slag from River Foulness sites